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Adding Fine Tuning To Standard Controls

If you have more than one VCO and have a transposer to offset them, you have probably experienced some difficulty in getting them perfectly tuned. The broad tuning range of the standard 500K pots in the 4781 can make fine tuning very sensitive and difficult to do rapidly. If you have such problems, there is an inexpensive and simple way to eliminate them. Whether or not you presently have a 4781 transposer, you can easily convert it or build from scratch a Fine Tuning Transposer.

The transposer consists of two 500K pots (there are four of these on a stock 4781), and two 10K pots, wired in series, with the front panel patch controls similar to a normal transposer, except that there are two less outputs. The upper fine tuning controls can be set to approximate midrange. Then the lower pots can be used to tune the interval desired, but don't be concerned with getting the VCO's perfectly tuned. Go back to the upper controls and fine tune the VCO's for perfect zero beat tuning. Eventually you will be able to tune the lower pots so the upper pots will be in the center of their ranges for zero beat tuning, thereby enabling minor corrections either up or down in pitch while playing with the other hand.

Editor's Addenda: Gary has a good idea here, but keep in mind that fine tuning can be used in other modules, too. For example, how about fine tuning on the keyboard pitch control? Also, check with your local electronic parts distributor to see if he stocks a system of "custom built" replacement pots. Rather than stock thousands of potentiometers, many dealers stock an assortment of potentiometer "bodies" with various resistances, various shaft lengths and styles, and rear mount add-on switches. If your dealer uses this system, have him make up a dual section potentiometer with concentric shafts. The front section of the pot (the section which will be directly behind the panel) should be a value equivalent to the pot which you are replacing for fine tuning (in the case of the 4781, this would be 500K). The rear section of the pot should be a value of approximately 10% of the original pot (this will be the fine adjustment). The use of concentric shafts will allow separate control of each pot section via two front panel knobs which appear to be stacked on top of each other. The rear or outer knob will control the front section of the pot, which in our case will be rough tuning. The front or inner knob of the control will adjust the rear pot section, which is our fine tuning. Use of this type of control will allow adding fine tuning to any control you wish, and you won't need to drill a hole for another pot. This keeps wasted space to a minimum. If dual section pots are used to add fine tuning to a 4781, you can still have four outputs on one transposer, and be able to fine-tune a 5 note chord.

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Polyphony - Copyright: Polyphony Publishing Company


Polyphony - Jul 1976

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Feature by Gary Miller

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